In a linear motor, a mover is moved linearly relative to a stator. In the stator of the linear motor, a plurality of permanent magnets is arranged so as the N and S magnetic poles are formed alternately. On the stator, the mover is arranged via a gap. In order to maintain the gap constant between the stator and the mover, linear movement of the mover is guided by a guide device such as a linear guide or a bearing.
In the mover, a magnetic body core is provided facing the permanent magnets. The core has a plurality of salient poles projecting toward a field magnet part. The plural salient poles are wounded with three-phase coils of U, V and W phases, respectively. When a three-phase AC (alternate current) having a phase difference of 120 degrees is passed through the three-phase coils of U, V and W phases, a moving magnetic field is generated in the three-phase coils. By the action of the moving magnetic field produced by the three-phase coils and the magnetic field produced by the permanent magnets, the mover moves linearly.
The core is provided in order to strengthen the magnetic field generated by the coils. The core is made of a magnetic material such as silicon steel. Therefore, even while current is not passed through the coils, magnetic attraction is generated between the salient poles of the core and the permanent magnets. When the mover moves along the stator, the salient poles of the core are attracted by front permanent magnets or attracted back by rear permanent magnets due to the magnetic attraction. Therefore, the magnetic attraction added to the mover varies periodically per magnetic pole pitch of permanent magnets. This periodic variation in attraction is called cogging. Even if a current is passed through the coils, there remains cogging, which acts as disturbance.
As an approach to cancel cogging, as illustrated in FIG. 11, there is known a linear motor having auxiliary magnetic poles 2a and 2b of magnetic bodies provided at respective ends in the moving direction of the core 1 of the mover (see Patent documents 1 and 2). In this linear motor, the auxiliary magnetic poles 2a and 2b are provided to strengthen the magnetic flux of salient poles 1a and 1b at respective ends in the moving direction of the core 1. If the auxiliary magnetic poles 2a and 2b are not provided, a magnetic circuit of the salient poles 1a and 1b at the respective ends is difficult to form and the magnetic flux of the salient poles 1a and 1b at the respective ends becomes weaker than that of the center salient pole 1c. When the magnetic flux of the salient poles 1a and 1b at the respective ends becomes weak, the magnetic flux of the salient poles 1a and 1b and the magnetic flux of the center salient pole 1c are unbalanced to cause cogging. The auxiliary cores 2a and 2b are provided to strengthen the magnetic flux of the salient poles 1a and 1b at the respective ends and solve the problem of unbalancing.    [Patent Document 1] Japanese Utility Model Laid-Open No. 7-53427    [Patent Document 2] Japanese Patent Application Laid-Open No. 55-68870